Magnetic microphone



May 15, 1951 s. F. LYBARGER 2,552,800

MAGNETI C MI CROPHONE Filed Deo. 2'7,` 1947 Mtl F0643 Patented May 15, 1951 UNITED STATESI PATENT OFFICE E. A. Myers & partnership Sons, Mount Lebanon, Pa., a

Application December 27, 1947, Serial No. 794,146

`This invention relates to small magnetic microphcnes, such as used in hearing aids.

Heretofore, magnetic microphones have been either too large and bulky, or too low in efliciency for satisfactory use in hearing aids. The cost of such microphones also has been a serious factor in limiting their use. Therefore, the `standard microphone for vacuum tube hearing aids has been the Rochelle salts crystal type. While eilicient and cheap, the crystal microphone has some serious disadvantages. First, the crystal is destroyed by temperatures in excess of about 120 F. Second, the crystal is slowly but surely damaged by the humidity conditions encountered in hearing aid use. Third, at the usual operating temperatures of about 90 F. the efficiency is appreciably lower, particularly at the low frequencies, than at usual room temperature.

It is among the objects of this invention to provide a magnetic microphone which is free from temperature and humidity failures; which has an efficiency essentially the same as that of a good crystal microphone; which is small and light-weight; which is sufficiently sensitive for hearing aid use; which has very exible and easily adjusted frequency response characteristics that are particularly suited to hearing aid use; which can be easily manufactured and adjusted and tested; and the output of which is not seriously aiected by temperature changes.

In accordance with this invention, a diaphragm adapted to be vibrated by sound waves is connected by a drive pin to a smaller and stiffer diaphragm which therefore is driven by the first diaphragm. A magnet-pole piece system which includes the usual permanent magnet and one or more pole pieces with coils wound on them, is spaced by an air gap from the small diaphragm. Electric voltage is generated in the coil or coils as the width of the air gap is changed by the vibrating small diaphragm. The large diaphragm, which receives the sound waves and has vhigh compliance, forms with a supporting case a relatively large chamber that does not reduce materially the compliance of the vibratory system. A1-

7 Claims. (Cl. 179--114) 4The magnetic system preferably is mounted in a second smaller case detachably connected to the outside of the back wall of the main case, the back wall being provided with an opening that is covered by the small diaphragm. The large chamber inside the main case may be coupled to the outside of the case by an accoustic inertance to increase the response in the low frequency portion of the range covered by the microphone.

The preferred embodiment of the invention is illustrated in the accompanying drawings vin which Fig. 1 is a side View of the microphone; Fig. 2 is an enlarged transverse section taken on the line II-II of Fig. l, with the magnetic system shown in plan; Figs. 3 and 4 are vertical sections taken on the lines III-III and IV-IV, respectively, of Fig. 1; Fig. 5 is a fragmentary transverse section showing a modified arrangement for the small diaphragm; and Fig. 6 is a graph showing two different response curves.

Referring to the drawings, the main cup shape case l of the microphone is made of some suitable metal, the choice of which will depend on the method of diaphragm clamping. The open front of the case is closed by a diaphragm 2 cemented to the front edge. I'his diaphragm preferably is formed from aluminum foil and is dished to strengthen it. Inside of the case there is a drive pin 3, the front end of which is cemented to the center of the diaphragm. The rear end of the pin is connected in a manner to be described presently to the center of a second diaphragm 4, preferably of magnetically permeable material, which is considerably smaller and stiffer than the other diaphragm. The back wall of the case has a central opening 6 encircled inside the case by a recess containing a circularseat 1 machined in the back wall. The small diaphragm rests freely against the seat to provide freedom from appreciable air gap changes with temperature, and is prevented from moving radially by the Wall of the seat recess. Machining of the case to give accurate spacing of the diaphragm from vthe cuter surface of the case is accomplished easily. The small diaphragm is locked in the recess by a circular cover plate 8 fastened to the back wall of the case by screws 9. The back side of the plate is provided with a circular recess of slightly less diameter than diaphragm 4 with which the plate forms a narrow cavity or chamber Il. A large chamber l2 remains between the plate and the large diaphragm.

Although the small diaphragm may itself serve as an armature, it is preferred to mount one o1 the simplest is a piece of silk the back walll of theY main case I -threaded in the plate 8. The front surface of the vflange 32 lies in a plane perpendicular to the faxes of both cases, and the Vfront endsof the posite sides of the diaphragm. By initially having the center portions of the disks bowed away from the diaphragm, their vedgeswill tightly engage it when they are flattened against the diaphragm. The use of two armatures prevents the diaphragm from being distorted by the clamping pressure. I

The plate 8 is provided with a central opening Y,

I1 through which drive pin 3 extends, but this opening is sealed by a very thin'sheet of rubber I8 that snugly engages the pin and'is cemented to the plate. This seal has very low acoustic 'transmissionfbut' is flexible enough to permit free vibrationof the drive pin. 'The plate is provided-with asecond opening I9 that is covered by suitable acoustic damping resistance material, of which several kinds are well-known. VOne of cemented across theV opening.

In orderto increase the low frequency response, a small tubeg22 having a length Vof the order 'of one-half inchj and a diameter of 33e' inch is supported in the large chamber I2 of thelcase `by a hollow angle member 23, the outer en d of which The small `'diaphragm 4 forms part of an Ielectro-magnetic generating system by which the movements of the diaphragms are converted 'into electrical energy. 'I'he rest of the generating system is formed from a magnet-pole-piece sys- .tem spaced by an air gap from the back side of the small diaphragm. This system includes a permanent magnet 26, preferably of some strong- .ly magnetic material, andV forwardly extending pole-pieces 21 of highly magnetically permeable material on which electric coils 28 are mounted. The front lends of the pole-pieces are spaced a short distance from the rear armature disk t3 Vto provide the usual air gap. This magnetic system preferablyv is connected by vscrews 20 to Vthe back wall of a small cup-shape case 3l that lhas an open front. The front edge of this case is surroundedby an Yintegral flange 32 through which extend screws 33 that also extend through and are pole-pieces project from the front of case 3I and yare accessible for accurate grinding in a plane Aparallel to and a constant distance from the front plane of the flange. Accurate adjustment 4of the air gapin front of the pole tips can be accomplished, when necessary, by inserting thin shims between the two cases, or by partial demagnetization of permanent magnet 26. The Y Ylatter method can be used only for. a very small range of accurate adjustment. One end of coils -28is grounded `to case 3l, and the other end 30 extends through the side wall of that case for connection in a hearing aid circuit.

Although the free Contact of the small diaphragm with seat 1 gives freedom from appreciable air gap changes with temperature, rattle may occur if the diaphragm is not perfectly flat or is not pulled against the seat with sumcient force by the permanent magnet. Therefore, it sometimes is desirable to clamp the edge of the diaphragm tightly between its seat and plate 8.

For this purpose, as shown in Fig. 5, the back side-of the plate is provided with an annular rib 36 which presses the edge of the diaphragm 'against'the seat. V'If maximum freedom from thermal Veffects is necessary, the material of case I and plate 8 should have the same coefiicient ofthermal expansion as the diaphragm t to prevent'reduction and possible closure of the air gap at low temperatures. It usually is desirable to use a magnetic diaphragm 4, and case I and plate' 8 also fmay. be made from the same magnetic material.' However, small case 3| should be `made of non-magnetic material to prevent serious shunting ofthe permanent magnet. As the small case is short in the direction perpendicular to the diaphragms, itsjexpansion Vand `contraction at a different rate than the polepieces and other parts has little effect on the air gap. By also providingY diaphragm liwith only oneV armature, as shown in Fig. 5, a considerably higher resonant frequency for the system can be obtained. When a single armature 3`I is used, it is soldered or Vcemented to the front of the diaphragm so that it will not be necessary to depend ,on riveting, which might distort the diaphragm, to hold the armature in place. The drive pin is rivetedv over the armature Vmerely to rigidly connect the pin to the armature and small diaphragm. Y

The primary purpose of this invention is to provide a high-output, small size magnetic microphone with frequency response characteristics suitable for hearing aid use. The response of such a microphone need not extend below a frequency of 250 to 400 cycles. This is advantageous because it reduces noise and prevents overloading from the `higher speech powers encountered at low frequencies. A definite need for moreamplication in the range from about 1500 to e000 cycles than in the region below this should also be kept kin mind. The effects of head diffraction and ear canal resonance give an individual who 'is not using a `hearing aid a considerable amount of amplication at the higher frequencies above about 1500 cycles. To give a net flat response curve with a hearing aid, some element in the 'system must provide selective amplification of increasing the desirability of added high frequency response inthe hearing aid to give balanced performance. The microphone of the present invention is particularly adapted to be Constructed with response characteristics suitable for vhearing aid use.

i The diaphragm 2 has a relatively large effective VIarea A1 (about 13/8" outside diameter is suitable) Inback of the large diaphragm isa cavity of acoustical compliance Cai coupled to the outside of case l by the tube 22 having an inertance MA1 and an acoustical resistance RM. The effect is to raise the low frequency response by some fteen decibels at some peak frequency. Generally, the resonant effect of tube 22 is made to be maximum at from 300 to 400 cycles, although it can be raised or lowered to reduce or extend the frequency range of the microphone. The cavity compliance Cin is also coupled to the small cavity Il, whose acoustic compliance is CA2, by the acoustic resistance RAZ formed by the silk A covering hole I9 in plate 8. The small diaphragm is also coupled to cavity Il since it forms one wall of it, and also it is coupled to the cavity in case 3 behind it, which has an acoustical compliance CAS. The acoustical compliance CA2 of cavity Il in front of the small diaphragm, taken in conjunction with the damping element RAe, can be used to control the damping of the principal peak in the microphone response. The magnetic system is coupled to the small diaphragm, and has the eifect of increasing the compliance C2 because of the action of the permanent magnet, and of increasing the effective mechanical resistance of the second or rear diaphragm system, depending on the electrical load placed acrossl the microphone coils. The output of the microphone depends on the alternating velocity of the second diaphragm system, and for a flat output this velocity should be approximately constant with frequency. For the shape of output curve desirable for hearing aid use, the velocity shouldbe roughly constant from 300 to 1000 cycles, and should rise to a higher level above this. Because of the increasing alternating velocity of the small diaphragm for a given amplitude of vibration at high frequencies, the response at frequencies y Curve A Curve B A1 sq. cm 5 5 C A, cvs units 3X10' 3Xl0' A,.-. sq. cm. 1.1 1.1 CAL." CgS Units 1.25)(107 0. 2X1()-7 C... cgs units.. 5 l07 5 10 NJIl-viq grams- 0.25 (l. 25 15.4 -s -s CHC, cnL/dyne-. GX10 6X1()` MM cgs units-. 3.8 3. 8 RM ogs units. 900 900 While the above values are representative of successful microphones made according to this invention, relatively large departures may be made from those values without affecting the scope of the invention. Values of as low as 2 10-8 cgs units have been successfully employed.

The curves illustrated in Fig. 6 were taken in conjunction with a miniature input transformer and represent the voltage available at the` grid of the rst amplifier tube in the usual hearing aid amplifier. Curve A is that of a microphone havlng high sensitivity with a moderate peak in. response between 1500 and 3000 cycles, at the principal resonant requency of the vibratory system. The amount of the peak response is limited by the acoustic resistance 20. At low frequencies, due to the action of' the tube 22, the response holds up down to about 300 cycles.

Curve B is that of a microphone very similar to the one of curve A. The volumeof cavity H, however, has been reduced, and its compliance lowered, resulting in a much greater damping action by the acoustic resistance 20, and flattening out the peak to a gently rounded form. This type of curve is particularly well suited for hearing aid use.

Accordingto the provisions of the patent statutes, I have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may he practiced otherwise than as specifically illustrated and described.

I claim:

1. A magnetic microphone comprising a diaphragm adapted to be vibrated by sound waves, a still* pin having one end connected to the diaphragm and driven thereby, a smaller and stiifer diaphragm driven by the pin, a magnet-polepiece system spaced by an air gap from the small diaphragm for generating electrical voltage as the width of said `air gap is changed by the vibrating small diaphragm, a case surrounding the space between the two diaphragms and supporting them, a plate mounted in the case between the two diaphragms and separating the space between them into a large chamber behind the large diaphragm and a small chamber in front of the other diaphragm, said plate being provided with an opening for passage of said pin, and a flexible sealing member engaging the pin and secured to the plate around it to seal said opening, said plate being provided with a second opening, and acoustic damping resistance material covering said second opening.

2. A magnetic microphone comprising a diaphragm adapted to be vibrated by sound waves, a pin having one end connected to the diaphragm and driven thereby, a smaller and stiifer diaphragm driven by the opposite end of the pin, a wall separating the space between the two diaphragms into two chambers of unequal size, the small diaphragm forming one side of the smaller chamber, said smaller chamber being sealed except for an opening through said wall, acoustic damping resistance material covering said opening, and a magnet-pole-piece system outside of said chambers and spaced by an air gap from the small diaphragm for generating electrical voltage as the width of said air gap is changed by the vibrating small diaphragm.

3. A magnetic microphone comprising a case having an open front and a central opening in its back wall, a diaphragm closing the open front of the case and adapted to be vibrated by sound waves, a pin in the case having its front end connected to the diaphragm and driven thereby, a smaller and stiifer diaphragm in the case engaging the inner surface of said back wall around said opening, the rear end of the pin 'being connected to the small diaphragm for driving it, a magnet-pole-piece system connected to the outside of said back wall and havingits pole-piece portion spaced by an air gap from the small diaphragm, a plate mounted in the case between the rattacco two' diaphragms Y'and dividing the space between 'them into a large chamber behind the large dia- Dhragm and a small chamber in front of the other diaphragm, the plate being provided with an opening for passage of said pin, exible material engaging the pin and secured to the plate around it to seal said opening,'the case being provided with a hole that opens into the space between the two diaphragms, and a small tube inside the case connected to said hole for increasing the low frequency response of the microphone, said plate being provided with asecond opening, and acoustic damping resistance material covering said second opening.

4. A magnetic microphone comprising a case having an open front and a central opening in its back Wall, a diaphragm closing the open front of the case and adapted to be vibrated by sound waves, a pin in the case having its front end connectedV to the diaphragm and driven thereby, a smaller and stiier diaphragm inthe caseengaging the inside of said back wall around said opening, the rear end of Vthe pin being connected to the small diaphragm for driving it, a plate fastened tothe inside of said back wall around said opening and spaced from at least the major portion of the smallv diaphragm to form a chamber in front of it, said plate having an'opening through it for said pin, flexible material engaging the pin and secured to the plate around it to seal said opening, said plate being provided with a second opening, acoustic damping resistance Inaterial covering the second opening, and a magnet-pole-piece system connected to the outside of said back Wall and having its pole-piece portion spaced by an air gap from the small diaphragm.

5. Amagnetic microphone comprising a `case having an open front and a central openingI in its back wall; the inside of said back wall being provided with a recess around said opening, a diaphragm closing the open front of the case and adapted to be vibrated by sound waves, a pin in th-e case having its front end connected to the diaphragm and driven thereby, a smaller and stiier diaphragm in said recess engaging said back wall around said opening, the rear end of the pin being connected to the small diaphragm -for driving it, a plate fastened to the inside of said back wall around said recess and having a recessed' back side of smaller diameter than the small diaphragm to provide a chamber in front of the small diaphragm, said plate having an openingI through it for said pin, a flexible sealing member engaging the pin and fastened .to the plate around it to seal said plate opening, the plate being provided with a second opening, acoustic damping resistance material covering said second opening, and a magnet-pole-piece system connected to the outside of said back wall and having its pole-piece portion spaced by an air gap from the small diaphragm.

6. A magnetic microphone comprising an elec- Ytro-magnetic generating system including a dial phrag-m, a drive pin connected to the diaphragm,

a larger and more iiexible diaphragm spaced from the other diaphragm and adapted to be 5 vibrated by sound waves, the large diaphragm being connected to the pin for driving it, a case surrounding the space between the two diaphragms and supporting said system, and a plate mounted in the case between the two diaphragms and dividing the space between them into a large chamber behind the large diaphragm and a Small chamber in front of the other diaphragm, said small chamber being sealed except for an acoustic resistance opening of very small size through the plate.

7. A magnetic microphone comprising a case having an open front and a central opening in its bac-k wall, a diaphragm closing the open front of the case and adapted to be vibrated by sound Waves, apin in the case having its front end con'- nected to the diaphragmand driven thereby, a smaller and stiffer diaphragm in the case engage ing the inside of said back Wall around said opening, the rear end of the pin being connected to the small diaphragm for driving it, a platefastened to the inside of the back wall around Ysaid opening and spaced from at least the major portion of the small diaphragm to form a small chamber in front of it, said plate having an opening through it for said pin, means sealing said plate opening, said plate being provided with a second opening, acoustic damping resistance material covering Said second opening, a second cup-like case having an open front surrounded by a ange, fastening members detachably connecting said flange to the outside of said back wall around said opening, and a magnet-pole-piece system mounted in said second case with the front face of its pole-piec-e portion lying in a plane parallel to the plane of the front face of said flange, said pole-piece portion being spaced by an air gap from the small diaphragm.

SAMUEL F. LYBARGER.

REFERENCES CITED The foliowing references are of record in the le of this patent:

UNITED STATES PATENTS DateV 

